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Heathkit AA-151 project. Help!

Discussion in 'Tube Audio' started by hugh811, Jun 14, 2018.

  1. hugh811

    hugh811 New Member


    I have been a while on the project to restore a Heathkit AA-151. With much fun!

    Some replacements already done: replaced old capacitors and resistors that are out of tolerance range; new el84 power output tube set; Deoxit used for pots.

    Other preamp tubes seem to be good according to my tube tester.

    It is much better now. It sounds good, but there is still a "slight" 60hz constant hum out of both speakers. Hum is even out of both speakers. I described it as "slight" as the amp is hooked up to a high sensitivity 16 Ohm JBL D130. The hum is "independent", constant at all volume levels.

    The old coupling capacitors still there, because they are still within tolerance range. I think that the problem lies in these coupling capacitors or big power filter capacitors.

    Please help me to identify the main suspect and what to try first. It could save time and labor hours.

    I thank you. Appreciate your help!
    Last edited: Jun 14, 2018


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  2. s-petersen

    s-petersen Scott Subscriber

    Middletown,New York
    Hum in both channels is usually the big filter cans, The coupling caps should be checked for leakage at their rated voltage, or just simply replaced, as they are a low cost item.
    drtool likes this.
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Ball Ground, GA
    Two things to check -- that the output tubes are balanced, and that the 6AU6 tubes don't have any heater to cathode leakage. Both will cause hum in that unit.


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